Updates and Corrections

[Qiaozhi]

You have absolutely right Qiaozhi, but the problem is that i dont trust anymore the Chinese suppliers, i have buy from these
7660A and LT1054, the 7660 dont burn because i have hold the voltage lower then 12, the 1054 burn at ....14vdc.
Because of the pins compatibility, im never shure what of kind the LT1054 are, maybe they are 7660a.

I purchase all my components from Farnell (Element 14). Then I know I'm not buying a fake.

[eclipse]

Looking at SURF PI it has the same arrangement for -5V so it should be working fine (connect pin 8 of 7660 to +5V), remove 7905

[Qiaozhi]

Looking at SURF PI it has the same arrangement for -5V so it should be working fine (connect pin 8 of 7660 to +5V), remove 7905

That won't work.

The power supply circuit for the MPP is completely different to both the Geotech Baracuda and the Surf-PI.
The MPP uses battery positive as the 0V reference. The -5V supply is generated directly from the battery negative via a 79L05 with respect to 0V, whereas the +5V supply uses the LT1054 as a voltage doubler to create a supply that is positive with respect to battery negative. This generates another voltage rail that is double the amplitude of the battery voltage with respect to battery negative (and effectively the same as the battery voltage but with respect to 0V). This voltage rail is used to create the +5V supply via a 78L05.

[eclipse]

Yes I realize that.. but I'm referring to PSU on the Geotech Baracuda on my posts and way to improve it (as this is what the title of this topic is).
10x 7660 cost 1$ on ebay, while LT1054 - single piece cost 3-4$. Yes probably it won't be possible to provide full -5V (going to be in the range of -4.5V to -4. or better but it's cheaper and more foolproof against over voltage neglect, also I saw it's being used in the commercial Surf PI project.

[homefire]

Using Three 18650 Li provides 11.1 V and solves the problems. Do you really think your going to notice the 1.0 V difference ? Hardly..

Even fully charged Li would only provide 12.6 V for the first few minutes. There is some overhead available on the 12V ratings. Using Genuine parts would not be a issue I think.

HF

[eclipse]

4x18650 give 16.5V when fully charged and it's more than even LT1054 can handle. It's what I use in most builds.

Also keep in mind that not all 7660 are created equal.
For example I've got one TC7660 from RS components which seems to rated only for 10V max.

Bottom line is: For P channel mosfet designs with -5V supply and 7660/1054 it's best to power the chip from fixed +5V
Saves a lot of headaches and probably quite a bit of cash to buy expensive 1054 with (almost) no drawbacks (disclaimer: not tested yet, but you can see this setup in use with Surf PI)